Conventional gel coat compositions, typically formulated from thermosetting resins such as unsaturated polyester, acrylate, and urethane type resins and combinations thereof are useful as the exterior paint layer for composite materials such as boat hulls, bath tub enclosures, and the like. A gel coat is a pigmented, filled, and pre-promoted resin (usually polyester) which is sprayed with an initiator onto molds from a high pressure spray gun to a film thickness of up to 0.75 mm. The film cures prior to reinforcement with glass fibers and laminating resins. The gel coat should exhibit low viscosity at high shear, should resist sagging, and produce a gel time of 8-15 minutes. For marine applications the gel coats should exhibit hydrolytic stability and good weatherability.
Unsaturated polyester resins are generally prepared by reacting in a glycol with an unsaturated acid and are then mixed with polymerizable diluents such as styrene and methyl methacrylate (MMA). The reactive diluents are also used to reduce resin system viscosity. When cured, the reactive diluents become a part of the resin system to produce a rigid cross-linked structure with desirable properties. Conventional unsaturated polyester resin usually contains 45%-35 weight % of reactive diluents and other volatile organic compounds (VOC).
The presence of large amounts of styrene and other VOC in such resin compositions results in the emission of styrene vapors into the work atmosphere which constitutes a hazard to workers and the environment. In view of this environmental hazard, governments have established regulations setting forth guidelines relating to volatile organic compounds which may be released to the atmosphere. The U.S. Environmental Protection Agency (EPA) has established guidelines limiting the amount of VOC released to the atmosphere, such guidelines being scheduled for adoption or having been adopted by various states of the United States. Guidelines relating to VOC, such as those of the EPA, and environmental concerns are particularly pertinent to the gel coat and other coating industry which use styrene or organic solvents and these VOC are emitted into the atmosphere.
To reduce styrene content and VOC in polymeric vehicles and formulated coating, researchers try to develop low VOC resin compositions in which VOC in the coating is kept at the lowest possible level. One way to reduce VOC is to reduce the molecular weight of the resin. According to polymer physics theory, the viscosity of polymers in the liquid state depends mainly on the average molecular weight, so it is desirable to reduce average molecular weight for low VOC product. Low molecular weight leads to a lower viscosity and lower styrene need. Compared with conventional resin, which has higher molecular weight and higher styrene content, the low VOC resin usually contains 35% or less styrene and VOC.
The lower molecular weight resin has the advantage of reduced VOC, but it also has disadvantages over the conventional resin. The coating or gel coat made with lower molecular weight resin tends to have poor properties in application compared to the conventional resin. While the conventional resin tends to form a tack-free curing surface, the coating or gel coat made with lower molecular weight resin tends to remain tacky for long periods of time after application. The surface tackiness is because of the oxygen inhibition on radical polymerization.
To get a non-tacky coating, for example, a film-forming material, such as paraffin wax may be included in the coating composition in order to prevent air inhibition and reduce the monomer vaporization. Paraffin or hydrocarbon waxes tend to migrate to the surface of the coating and serve as a film which reduces oxygen penetration at the coating surface. However, the presence of wax on coating surface will reduce secondary adhesive properties when laminate is put onto the coating later.
Low VOC and performance are incompatible characteristics with each other. The improvement of the poor performance and obtaining a tack-free tends to impair the low VOC property. There is a difficulty in attaining both low VOC and good application property.
It is desirable, particularly in view of the toxicity of the styrene monomer and government regulations, to reduce the concentration levels of the styrene monomer from the usual 40% to 50% by weight of the polyester resin to below 35% or 30% by weight. However, reductions in styrene monomer concentrations present problems in the polyester resins. The resin viscosity increases at lower monomer content causing difficulties in applying the resins, such as poor spray property and glass roll-out when the resin is sprayed or used in conjunction with glass fibers. In addition, the physical properties of the styrene monomer-reduced polyester resins are also greatly reduced without the use of additional, supplemental cross linkers in the polyester resin. Acrylic monomers, such as ethylene glycol dimethacrylate have been added to low or reduced styrene monomer polyester resins for marine grade gel coats and for outdoor applications. However, the result resin is hard to use because of a tacky surface.
In addition, in unsaturated polyester resin an aromatic diacid, such as isophthalic acid, is generally present to help improve the hydrolysis resistance of the film. However, the presence of the aromatic nuclei causes poor exterior durability to the coating film. The same problem is also presented in vinyl ester resins. Low VOC vinyl esters based on aromatic polyepoxide resin reacted with unsaturated monocarboxylic acid and other moieties are reported in U.S. Pat. No. 6,900,276. While the content of styrene is lower, the presence of the aromatic nuclei leads to unacceptable exterior durability. The vinyl ester based on aliphatic polyepoxides show poor hydrolysis resistance. Both of these vinyl ester resins are not suitable for gel coats requiring good hydrolytic stability and weatherability.
It is therefore desirable to provide new and improved, crosslinkable resin compositions to provide unsaturated, cured resins with better physical and chemical properties and to provide an unsaturated resin composition having reduced styrene monomer at the same time. Several approaches for addressing these limitations have been described in the prior art. These approaches include making polyester modifications using epoxy chemistry, urethane chemistry and acrylic chemistry.
Acrylic resin without aromatic nuclei show good properties and are developed for many applications including gel coat. Some acrylic base resins have been reported for gel coat formulation. Acrylic modified unsaturated resins are taught in EP708157. U.S. Pat. Nos. 4,742,121 and 6,492,470 teaches acrylic resins for gel coat comprising a vinyl monomer and a polyacrylate with various pendant unsaturated group.
The acrylic resins with urethanes have been reported via various techniques. U.S. Pat. Nos. 5,777,053 and 7,150,915, JP59157074, EP0254232, and WO2004014978 teach the acrylic polymers which contain urethane groups and are cross-linkable by vinyl polymerization. The urethane groups are introduced by reacting hydroxyl group pendant from the acrylate or polyester backbone with polyisocyanate.
WO2004056930 reported UV-curable epoxy acrylates. JP60123478 reported the isocyanuric ring-containing acrylate resin. Thermosetting acrylic resins were also synthesized with the reaction of polyacid and unsaturated epoxide such as glycidyl methacrylate. U.S. Pat. Nos. 4,774,267, and 4,831,066 reported the halfester of polyol and acid anhydride was esterified with glycidyl methacrylate for a photocurable dental material. JP63113011 and JP2001011153 reported ink or UV-cure coating composition made by the reaction of halfester of glycol and acid anhydride with glycidyl methacrylate.
While these approaches have led to improvements in hydrolytic stability and durability, none of these solutions to the problem arising from low viscosity and poor application properties have been totally satisfactory. There remains a significant need for thermosetting resin which has better cured product, especially in the case of low VOC resins, which contain relatively low volatile vinyl monomers, and with outstanding weather resistant.
It is an object of the present invention to provide a novel cross-linkable resin composition which avoids the problems of high VOC and provide gel coat which can be used in a wide variety of applications. It is a further object of the present invention to provide a cross-linkable polyester acrylic resin composition which can be formulated to a high solids gel coat composition with excellent weather, gloss and water resistance enabling the gel coat composition to be used as low VOC gel coats or in-mold coating system, particularly for use as an gel coat for boat and automobiles.
The above deficiencies of gel coats have been largely overcome with the gel coats based on the polyester acrylic resins of the present invention. The new polyester acrylic gel coats show low viscosity and excellent hydrolytic and weathering stability.